Isomerization processes generate a mixture of isomers that usually require separation and recycle of the non-isomerized components. For example, the effluent of a paraffin isomerization reactor may contain normal alkanes, monomethyl-branched alkanes, ethyl-pentane, and multimethyl-branched alkanes. Traditionally, only the normal alkanes would be separated from the mixture by adsorption and recycled to the isomerization reactor; see U.S. Pat. No. 5,043,525, and any monomethyl-branched alkanes would be collected with the multimethyl-branched alkanes as product. However, it is the multimethyl-branched alkanes that are the most desired and have the highest octane number components. Therefore, the more efficient approach would be to adsorptively separate only the multimethyl-branched alkanes as product and recycle the normal and the monomethyl-branched alkanes to the isomerization reactor. However, with many of the adsorbents available today this separation is extremely difficult because the adsorbents exhibit only a slightly greater selectivity for the monomethyl-branched alkanes as compared to the multimethyl-branched alkanes, while the selectivity for the normal alkanes is greater relative to the monomethyl- and multimethyl-branched alkanes. Desorbents capable of fully desorbing the normal alkanes also prevent the monomethyl-branched alkanes from being adsorbed and separated from the multimethyl-branched alkanes. The invention described herein provides a process for separating the multimethyl-branched alkanes from an isomerization reactor effluent using a single adsorbent contained in a simulated moving bed and two desorbents of differing desorbent capability.
Others have used two desorbents in simulated moving bed adsorptive separations. U.S. Pat. Nos. 4,031,151, 4,031,156, and 4,031,155 disclose using a stronger desorbent in the desorption zone and a weaker desorbent in the rectification zone. The improvement provided by these disclosures center on effecting the desorption of the weakly adsorbed raffinate components in the rectification zone so that when the rectification zone becomes the desorption zone in a succeeding cycle of operation, only the desired sorbate component remains adsorbed.
U.S. Pat. Nos. 4,036,745 and 4,006,197 describe using a first desorbent to desorb surface adsorbed aromatic contaminates from the adsorbent and to sweep the contaminates out of the simulated moving bed and then a second desorbent to remove and collect the desired component from the pores of the adsorbent. U.S. Pat. No. 3,723,302 discloses a two-desorbent system for the removal of contaminants in a process for separating olefins from paraffins. The first desorbent causes contaminants to be desorbed and removed in a first extract stream and the second desorbent is used to desorb the product which is removed in a second extract stream.
U.S. Pat. No. 3,696,107 discloses a process for separating para-xylene from a mixture of C.sub.8 aromatics where a first desorbent is used to desorb para-xylene from the adsorbent and a second desorbent is used to sweep the desorbed para-xylene from the interstitial void spaces between the adsorbent particles. U.S. Pat. No. 5,510,564 discloses a process for separating normal paraffins and isoparaffins along with the removal of aromatics. Paraffins and the aromatics are sorbed by a sorbate and isoparaffins are collected. The paraffins and aromatics are desorbed using a first desorbent and are contacted with a second sorbate which sorbs the aromatics. The normal paraffins are flushed or purged from the interstitial void spaces between the second sorbate particles and the adsorbed aromatics are then desorbed.
U.S. Pat. No. 3,205,166 discloses a separation process producing a normal aliphatic hydrocarbon portion, an aromatic component portion, and a branched chain and cycloparaffinic hydrocarbon portion. The process uses a mixture of adsorbents, one selective for normal aliphatic components and the other selective for aromatic components. A first desorbent is used to desorb the normal aliphatic components from the first adsorbent, and a second desorbent is used to desorb the aromatic components from the second adsorbent. Multiple adsorbents are also disclosed in U.S. Pat. No. 5,405,534 describing an apparatus for separating a three-component mixture.
U.S. Pat. Nos. 5,530,173 and 5,530,172 describe concurrent isomerization with separation of reactants and products using a simulated moving bed containing both catalyst and adsorbent. An embodiment disclosed is one where the desorbent used is itself isomerized thereby yielding two desorbents within the simulated moving bed.
Using two desorbents of different desporptive capacities enables the present invention to efficiently accomplish a difficult separation using a simulated moving adsorbent bed. A single desorbent having sufficient desorption capacity to desorb the normal alkanes would prevent separation of the monomethyl-branched alkanes from the multimethyl-branched alkanes. A single desorbent of low enough desorption capacity to effect the separation of the monomethyl-branched alkanes from the multimethyl-branched alkanes would be required in such high quantity to desorb the normal alkanes so as to make its use impractical.